1. Field of Invention
The present invention relates to a radar system and related method; and more particularly to a radar system and method, having the ability to vary the tracking range.
A continuous wave radar (CW) and a high pulse repetition frequency (PRF) radar are similar in that a high PRF in its basic form is a CW radar pulsed on and off at a high rate, such as several hundred kilohertz, for example; and the receiver is protected from direct transmitter leakage by a switch which is opened during the transmit period. However, in practice high PRF pulse Doppler (PD) radars are far more complex and are more effective against low flying aircraft in a heavy clutter background, in that they can discriminate more effectively between ground clutter and moving targets on a Doppler frequency basis. High average power can also be transmitted in that such system typically have approximately a 50% transmission duty factor; that is, the duration of the waveform is approximately the same as the interpulse period.
A linear FM waveform which is generated and compressed by a compression filter is sometimes referred to as a chirp waveform and is relatively simple to generate. Stretching and compression of a linear FM waveform is accomplished to retain the detection capability of a waveform of long duration while at the same time retaining the range resolution of a short pulse. A stretched waveform is generated actively by oscillator tuning a digital synthesis; and is demodulated by the first local oscillator and processed via Fast Fourier Transform. The waveform is generated from a narrow linear FM waveform by changing the relative phases of the frequency components of the waveform by a filter that distorts the phase; thus producing a stretch waveform, which is the waveform that is transmitted. The received echo is processed in a compression filter that readjusts the relative phases of the frequency components so that a narrow linear FM chirp signal is received.
One ranging method used with CW and high PRF PD radar is multiple PRF ranging which uses a moderate duty factor of one tenth or less and multiple range gated receivers. The receivers are sequentially opened for a period equal to the transmitted pulse width. A target will fall in only one or two receiver channels, which is an ambiguous range measurement; and ambiguity is resolved by cycling through several PRF's.
Another method of ranging a CW or high PRF PD radar is accomplished by applying linear frequency modulation (FM) to the microwave oscillator of the system for a short period during each transmit cycle. This application of FM modulates both the transmitted frequency and receiver local oscillator frequency, which produces a frequency or Doppler shift in the return signal that is range dependent. Thus, target signal frequency is the sum of the range frequency and Doppler frequency. By comparing with the target signal frequency when no FM is applied, the range component can be computed and displayed.
In a typical CW or high PRF linear FM Doppler radar system, a stretched waveform of a bandwidth .DELTA.F is transmitted during a time period .DELTA.T. During the receive cycle, the FM is removed by an identical .DELTA.F, .DELTA.T from the return signal. The target range where a radar return falls that is synchronous with the sweep time .DELTA.T generates a CW signal of duration .DELTA.T at an intermediate center frequency F0. Thus, the range resolution is proportional to the reciprocal of the bandwidth .DELTA.F in accordance with the following equation. ##EQU1##
Where;
K1=system processor FFT Weight widening PA1 K2=widening due to return signal nonsynchronous with sweep, PA1 dR=range resolution, PA1 .DELTA.F=transmit bandwidth. PA1 c=Velocity of light
The minimum range is determined by the transmit time .DELTA.T. Thus, varying the tracking range can be achieved by varying .DELTA.T while retaining .DELTA.F at the same value. If the tracking range is to be reduced, the resolution dR in equation 1.0 remains constant, but the FM slope is increased given a constant receiver IF bandwidth. This, however creates a waveform synthesis problem. It is difficult to reduce .DELTA.T by an appreciable factor, and still sweep the relatively wide .DELTA.F bandwidths usually required. If constant range coverage were required, a varying IF bandwidth and digital sampling rate change would be necessary, which is cumbersome. Such a stretch system also limits the minimum range resolution and requires an increase in the slope of the FM waveform.